1. Field of the Invention
The present invention generally relates to a method for controlling a laser and, more specifically, to a method for controlling the output of a vertical cavity surface emitting laser (VCSEL) by adding a time varying signal to a preselected portion of a digital data signal in order to be able to control the operation of the laser by monitoring the average power of a portion of the light output from the laser.
2. Description of the Prior Art
In certain applications of lasers, it is necessary to monitor the power of the light output from the laser. In edge emitting lasers, this function is performed by providing a photodiode to receive a portion of the light output from the edge emitting laser. Typically, the monitor photodiode is placed at a location opposite to that in which the operational light output of the laser is directed. If the laser is a vertical cavity surface emitting laser (VCSEL), this monitoring function is more difficult. Since light is emitted from a surface of the laser structure and not its edges, a portion of the light must be reflected back toward a light sensitive component such as a photodiode. The reflection of a preselected ratio of the light output from the laser can be accomplished by using a lens that is partially mirrored so that a portion of the light output from the laser is reflected back from the lens and falls upon a photodiode.
As will be described in greater detail below, high speed digital signals present a particular problem in terms of monitoring the output of a VCSEL. In a high speed digital signal, the rise and fall of the magnitude of electrical current passing through the VCSEL occur at such a rapid rate that the monitoring photodiode is unable to provide a monitoring signal that is sufficiently responsive to yield information regarding the depth of the signal (i.e. the difference in magnitude between the on pulses and the off pulses) or the bias level of the signal. This inability to track the rapid pulses with a photodiode is further complicated by the complicated manner in, which the bias level and depth magnitude of the signal can interact with each other to change their individual magnitudes without actually changing the average power level of the signal which is the only variable that can be directly monitored by the photodiode.
It would therefore be significantly beneficial if a method of controlling a VCSEL could be developed in which the average power monitoring signal provided by a photodiode could be sufficient to permit the control system to determine when either the bias level or depth magnitude of the digital signal is improper and requires correction. In addition, it would be significantly beneficial if this controlling method could allow both the bias level and depth magnitude of the signal to be corrected during operation without adversely affecting the accurate correspondence between the light output pulses from the laser and the electrical input pulses received by the laser.